This invention relates to a clock spring used with, for example, a steering apparatus of a motor vehicle to establish electrical connection between a stationary member and a movable member by using cables.
A clock spring is a device for connecting a stationary member and a movable member rotatably fitted to the stationary member through a cable. For example, clock springs of this kind have been used as electrical connection devices for steering apparatuses of motor vehicles.
A type of conventional clock spring is known which has a flat cable having a plurality of threads of conductors sandwiched in a pair of laminated band-like films. The flat cable is loosely accommodated in a coiled state in a cable housing provided between a stationary member and a movable member. One end of the flat cable is fixed to an outer cylindrical wall formed on one of the stationary and movable members, and the other end is fixed to an inner cylindrical wall formed on the other of the stationary and movable members.
In this clock spring, when the movable member is rotated, the flat cable accommodated in the cable housing can be wound around the inner cylindrical wall or unwound toward the outer cylindrical wall according to the direction of rotation of the movable member. No substantial tensile force is applied to the flat cable in a range between the state in which the flat cable is fully wound around the inner cylindrical wall and the state in which it is fully unwound toward the outer cylindrical wall. It is thereby possible to constantly maintain the electrical connection between the stationary and movable members rotated relative to each other.
In the above-described clock spring, since the flat cable is coiled or uncoiled by utilizing the difference between the diameters of the outer and inner cylindrical walls, the length of the flat cable to be used can be reduced if the difference between these diameters is increased, provided that the extents of rotation of the movable member are constant. However, the diameter of the inner cylindrical wall is determined by the diameter of a rotating shaft to which the clock spring is to be attached, e.g., the steering shaft of a vehicle, while the diameter of the outer cylindrical wall cannot be increased substantially, because it is desirable to reduce the overall size of the device. The difference between the diameters of the outer and inner cylindrical walls is therefore limited. Consequently, a very long flat cable is generally required for this kind of clock spring, and the total cost of the clock spring is increased because there is a difficulty in manufacturing such a long flat cable as is well known.